GB2070377A

GB2070377A - Amplifier arrangement

Info

Publication number: GB2070377A
Application number: GB8105460A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1980-02-25
Filing date: 1981-02-20
Publication date: 1981-09-03
Also published as: IT8119905A0; SG22084G; FR2476936B1; DE3106528A1; JPH0158684B2; CA1149029A; DE3106528C2; US4409558A; GB2070377B; IT1135576B; FR2476936A1; JPS56132003A; NL8001116A

Description

1 GB 2 070 377 A 1

SPECIFICATION

Amplifier arrangement The invention relates to an amplifier arrangement. A known amplifier arrangement comprises an input and an output, a first and a second transistor of the same conductivity type, whose base electrodes are interconnected, the emitter electrode of the first transistor being connected to the input, the emitter of the second transistor being connected to a first common point via a first resistor, the collector of the first transistor being connected to a quiescentcurrent source, and the collector of the second transistor being connected to the output and, via a second resistor, to a power-supply terminal.

Such a known amplifier arrangement is inter afia employed in SOAR protection circuits in integrated power amplifiers as is known interalia from "Neues aus der Technik", No. 3,1977, article number 406, page 4.

In this circuit arrangement a voltage which is a measure of the current, voltage and/or power load of an output transistor in the power amplifier is applied to the input. The amplifier circuit is then proportioned so that at the critical point, i.e. the point at which the protection circuit should be actuated, the voltage across the first resistor is equal to the voltage on said input. At this point the combination of the first and the second transistor, the first transistor in this known circuit being arranged as a diode; operates as a current mirror through the first resistor a current equal to the current supplied by the quiescent current source will flow and produce a voltage across said resistor, to which a circuit can respond which limits the drive of the protected output transistor.

Especially in an integrated circuit with p-type substrate, which is generally employed in practice, the first and the second transistor, when they are pnp-transistors, will be of the lateral type, so that they will have a comparatively low current gain factor, which is moreover subject to a substantial spread owing to process variations. As a result of this, the collector current of the second transistor at said actuating point will not be equal to the collector current of the first transistor and will be greatly dependent on process variations, so that the point at which the protection circuit is actuated is subject to a substantial spread. It is highly undesirable, because 115 in order to obtain a maximum output power of an integrated circuit the protection circuit of the output transistor should not be actuated until an extreme value is reached.

It is the object of the invention to provide an 120 amplifier arrangement in which for a voltage on the input at which the voltage across the first resistor is equal to the voltage on said input, the voltage on the output depends on the current gain factor of the first and the second transistor to a minimal extent.

According to the present invention there is provided an amplifier arrangement comprising an input, an output, a first and a second transistor of the same conductivity type, whose base electrode are interconnected, the emitter electrode of the first transis- tor being connected to the input, the emitter of the second transistor being connected to a first common point via a first resistor, the collector of the first transistor being connected to a quiescent-current source, the collector of the second transistor being connected to the output and, via a second resistor, to a power-supply terminal, and a base drive circuit having an input, which is connected to the collector electrode of the first transistor, a voltage follower output, which is connected to the base electrodes of the first and the second transistor, for driving said base electrodes in such a way that the current in the first transistor is equal to the quiescent current from said quiescent-current source, and a base-current diverting output, on which at least a part of the base currents of the first and the second transistor appears, said base-current diverting output being connected to a tapping on the second resistor at such a location that the voltage across the second resistor depends on the current gain factors of the first and second transistor to a minimal extent when the voltage on the input, which is connected to the emitter of the first transistor, has such a value that said voltage corresponds to the voltage across the first resistor.

Owing to the drive at a tapping of the second resistor a variation of the current gain factor of the two transistors, which variation results in a variation in the collector current of the second transistor, produces an opposite variation of the current applied to the tapping of the second resistor, which in the case of a correct location of said tapping, which may for example be determined experimentally, leads to a substantially full compensation at the said actuation point.

The present invention will now be described, by way of example, with reference to the single figure of the accompanying drawing which shows an embodiment of the amplifier arrangement in accord- ance with the present invention. In this drawing transistor Tp is the power transistor to be protected. The collector lead of the transistor Tp includes a resistor 5 for measuring the collector current of the transistor Tp, which resistor is generally selected to have a very small value and may for example be constituted by the resistance of a connecting track or a connection point. Between the collector and the emitter of transistor Tp a voltage divider with the resistors 6 and 7 is included for measuring the collector-emitter voltage of the transistor Tp. The junction point of the resistors 6 and 7 is connected to input A of the protection circuit and by a suitable choice of the values of resistors 5, 6 and 7 the voltage VA on point A can be an optimum function of the current through, and the voltage across, the transistor Tp to be protected for current, voltage and power limitation. The protection circuit comprises an amplifier, comprising pnp-transistors T1 and T2 whose base electrodes are interconnected. The emitter electrode of transistor T1 is connected to input A, whilst the emitter of transistor T2 is connected to the power supply terminal +VE; via a resistor 1. The collector of transistor T1 is connected to a quiescent-current source 4 with a current 1, whilst the collector of transistor T2 is connected to an GB 2 070 377 A 2 output B and, via resistors 2 and 3, to a negative supply voltage terminal -V, The base electrodes of the transistors T1 and T2 are connected to the emitter electrodes of a transistor 3, whose base electrode is connected to the collector electrode of transistor T1 and whose collector electrode is connected to the junction point between resistors 2 and 3. Transistor T3 drives transistorT, in such a way that it carries the current 1, supplied by the current source 4. Output B leads to the base of transistor T4, which is arranged as a differential pair with a transistor Ts. The base electrode of the transistor T5 is connected to a point C at reference voltage V,f. The collector electrode of the transistor T4 is connected to the base electrode of the transistor Tp to be protected.

Resistor 1 is dimensioned so that when the voltage VA reaches a value at which transistor Tp is to be protected, transistors T1 and T2 both carry the current 1. supplied by the current source 4 (ignoring the base current loss). Resistors 2 and 3 are then dimensioned so that the voltage VB on point B is then equal to the reference voltage Vref. At the point where the protection circuit is actuated the current, which is supplied to transistors T4 and T5 by the common emitter current source 8, is transferred from transistor T5 to transistor T4 and via the collector of transistor T5 is diverted from the base electrode of the transistor Tp to be protected, whose drive is limited at said protection point.

The current gain factor P of the lateral pnp- 95 transistors T1, T2 and T3 exhibits a spread in respect of process variations and moreover is greatly depen dent on the value of the bias current],, which may exhibit a spread as a result of a process variations. In practice this may give rise to a variation of the P between for example 3 and 30 in integrated circuits whose integration process is suitable for compara tively large currents, which may lead to a deviation in the collector current of transistor T2 relative to the current 1. of maximum 15%, which in its turn leads to an equal error in the voltage V13, i.e. the value of the voltage VA for which the drive of transistor T, is limited. For the protection of transistors in integrated power amplifiers this is an impermissible variation and in practice means that transistor Tp is limited to a dissipation which is 15% smaller than the optimum value.

This problem can be mitigated by feeding the drive of transistors T1 and T2 via transistor T3, or alternatively a transistor arrangement comprising a plurality of transistors, and the collector current of the transistor T3, or the output current of such a combination, to a tapping between resistors 2 and 3, so that said collector current produces an extra voltage on point B. A decrease of the current gain factor of the transistors T1 and T2 causes a decrease of the collector current of the transistor T2 and at the same time an increase of the collector currents of the transistor T3, so that by a suitable choice of the ratio of the values of the resistors 2 and 3 the effect of a variation in 0 on the voltage VB is largely cancelled.

Claims

1. An amplifier arrangement comprising an in- put, an output, a first and a second transistor of the same conductivity type, whose base electrodes are interconnected, the emitter electrode of the f irst transistor being connected to the input, the emitter of the second transistor being connected to a first common point via a first resistor, the collector of the first transistor being connected to a quiescentcurrent source, the collector of the second transistor being connected to the output and, via a second resistor, to a power-supply terminal, and a base drive circuit having an input, which is connected to the collector electrode of the first transistor, a voltage follower output, which is connected to the base electrodes of the first and the second transistor, for driving said base electrodes in such a way that the current in the first transistor is equal to the quiescent current from said quiescent- current source, and a base current diverting output, on which at least a part of the base currents of the first and the second transistor appears, said base-current diverting output being connected to a tapping on the second resistor at such a location that the voltage across the second resistor depends on the current gain factors of the first and the second transistor to a minimal extent when the voltage on the input, which is connected to the emitter of the first transistor, has such a value that said voltage corfesponds to the voltage across the first resistor.

2. An amplifier arrangement constructed and arranged to operate substantially as hereinbefore described with reference to the accompanying drawing.

Printed for Her Majesty's Stationery Office by Croydon Printing Company limited, Croydon, Surrey, 1981. Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

C